Evidence of a resource trade-off between growth and chemical defenses among Caribbean coral reef sponges

Like all organisms, sponges allocate resources to life functions such as growth and reproduction. Additionally, some sponges are defended by secondary metabolites that deter potential predators. Assuming resources are limiting, species that produce defensive metabolites should allocate fewer resources to growth and reproduction. To test the hypothesis that there is a trade-off between chemical defense and growth, predator exclusion experiments were conducted to compare the growth rates of 7 common Caribbean sponge species with similar branching morphologies: chemically undefended species Callyspongia armigera, Iotrochota birotulata and Niphates erecta, and defended species Amphimedon compressa, Aplysina cauliformis, Aplysina fulva and Ptilocaulis walpersi. Thirty field experiments lasting 124 to 195 d were performed over a 9 yr period on a total of 1158 sponges. A 3-factor ANOVA was used to compare the effects of chemical defense (undefended/ defended), treatment (uncaged/caged) and season (summer/winter). Despite the fact that different sponge species could be allocating resources differently to reproduction, thereby potentially obscuring the interaction between growth and chemical defense, growth in cages was significantly greater for undefended than defended sponges (110.4 versus 65.8% growth yr–1). While the growth of chemically defended sponges inside and outside of cages was not different, growth of undefended sponges inside cages was significantly greater than outside cages, confirming that palatable sponge species were grazed by sponge-eating fishes. Growth during winter months was significantly less for both undefended and defended sponges compared to growth during summer months, but again, growth was greater for undefended sponges than for defended sponges. Differences in growth rates demonstrate that sponge species have evolved alternative allocation patterns to cope with resource constraints, as well as predation, and provide evidence that there is a metabolic cost associated with chemical defenses.